The conversion of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) to mevalonate is an early and rate-limiting step in the cholesterol biosynthetic pathway. This step is catalyzed by the enzyme HMG-CoA reductase. Statins inhibit HMG-CoA reductase from catalyzing this conversion. As such, statins are collectively potent lipid lowering agents.
Atorvastatin calcium, disclosed in U.S. Pat. No. 5,273,995, which is incorporated herein by reference, is currently sold as LIPITOR® having the chemical name [R—(R*,R*)]-2-(4-fluorophenyl)-β,δ-dihydroxy-5-(1-methylethyl)-3-phenyl-4-[(phenylamino)carbonyl]-1H-pyrrole-1-heptanoic acid calcium salt (2:1) trihydrate.
Atorvastatin calcium is a selective, competitive inhibitor of HMG-CoA reductase. As such, atorvastatin calcium is a potent lipid lowering compound and is thus useful as a hypolipidemic and/or hypocholesterolemic agent.
A number of patents have issued disclosing atorvastatin, formulations of atorvastatin, as well as processes and key intermediates for preparing atorvastatin. These include: U.S. Pat. Nos. 4,681,893; 5,273,995; 5,003,080; 5,097,045; 5,103,024; 5,124,482; 5,149,837; 5,155,251; 5,216,174; 5,245,047; 5,248,793; 5,280,126; 5,397,792; 5,342,952; 5,298,627; 5,446,054; 5,470,981; 5,489,690; 5,489,691; 5,510,488; 5,686,104; 5,998,633; 6,087,511; 6,126,971; 6,433,213; and 6,476,235, which are herein incorporated by reference.
Additionally, a number of published International Patent Applications and patents have disclosed crystalline forms of atorvastatin, as well as processes for preparing amorphous atorvastatin. These include: U.S. Pat. No. 5,969,156; U.S. Pat. No. 6,121,461; U.S. Pat. No. 6,605,729; WO 00/71116; WO 01/28999; WO 01/36384; WO 01/42209; WO 02/41834; WO 02/43667; WO 02/43732; WO 02/051804; WO 02/057228; WO 02/057229; WO 02/057274; WO 02/059087; WO 02/072073; WO 02/083637; WO 02/083638; WO 03/050085; WO 03/070702; and WO 04/022053.
Atorvastatin is prepared as its calcium salt, i.e., [R—(R*,R*)]-2-(4-fluorophenyl)-β,δ-dihydroxy-5-(1-methylethyl)-3-phenyl-4-[(phenylamino)-carbonyl]-1H-pyrrole-1-heptanoic acid calcium salt (2:1). The calcium salt is desirable, since it enables atorvastatin to be conveniently formulated in, for example, tablets, capsules, lozenges, powders, and the like for oral administration.
The process by which atorvastatin calcium is produced needs to be one which is amenable to large-scale production. Additionally, it is desirable that the product should be in a form that is readily filterable and easily dried. Finally, it is economically desirable that the product be stable for extended periods of time without the need for specialized storage conditions.
Furthermore, it has been disclosed that the amorphous forms in a number of drugs exhibit different dissolution characteristics, and in some cases different bioavailability patterns compared to the crystalline forms (Konno T., Chem. Pharm. Bull., 1990; 38; 2003-2007). For some therapeutic indications, one bioavailability pattern may be favored over another.
In the course of drug development, it is generally assumed to be important to discover the most stable crystalline form of the drug. This most stable crystalline form is the form which is likely to have the best chemical stability, and thus the longest shelf-life in a formulation. However, it is also advantageous to have multiple forms of a drug, e.g. salts, hydrates, polymorphs, crystalline, and noncrystalline forms. There is no one ideal physical form of a drug because different physical forms provide different advantages. The search for the most stable form and for such other forms is arduous and the outcome is unpredictable.
The successful development of a drug requires that it meet certain requirements to be a therapeutically effective treatment for patients. These requirements fall into two categories: (1) requirements for successful manufacture of dosage forms, and (2) requirements for successful drug delivery and disposition after the drug formulation has been administered to the patient.
There are many kinds of drug formulations for administration by various routes, and the optimum drug form for different formulations is likely to be different. As mentioned above, a drug formulation must have sufficient shelf-life to allow successful distribution to patients in need of treatment. In addition, a drug formulation must provide the drug in a form which will dissolve in the patient's gastrointestinal tract when orally dosed. For oral dosing in an immediate release dosage form, such as an immediate release tablet, capsule, suspension, or sachet, it is generally desirable to have a drug salt or drug form which has high solubility, in order to assure complete dissolution of the dose and optimal bioavailability. For some drugs, particularly low solubility drugs or poorly wetting drugs, it may be advantageous to utilize a noncrystalline drug form, which will generally have a higher initial solubility than a crystalline form when administered into the gastrointestinal tract. A noncrystalline form of a drug is frequently less chemically stable than a crystalline form. Thus, it is advantageous to identify noncrystalline drug forms which are sufficiently chemically stable to provide a practical product which is stable enough to maintain its potency for enough time to permit dosage form manufacture, packaging, storage, and distribution to patients around the world.
On the other hand, there are dosage forms which operate better if the drug form is less soluble. For example, a chewable tablet or a suspension or a sachet dosage form exposes the tongue to the drug directly. For such dosage forms, it is desirable to minimize the solubility of the drug in the mouth, in order to keep a portion of the drug in the solid state, minimizing bad taste. For such dosage forms, it is often desirable to use a low solubility salt or crystalline form.
For controlled release oral or injectable, e.g, subcutaneous or intramuscular, dosage forms, the desired drug solubility is a complex function of delivery route, dose, dosage form design, and desired duration of release. For a drug which has high solubility, it may be desirable to utilize a lower solubility crystalline salt or polymorph for a controlled release dosage form, to aid in achievement of slow release through slow dissolution. For a drug which has low solubility, it may be necessary to utilize a higher solubility crystalline salt or polymorph, or a noncrystalline form, in order to achieve a sufficient dissolution rate to support the desired drug release rate from the controlled release dosage form.
In soft gelatin capsule dosage forms (“soft-gels”), the drug is dissolved in a small quantity of a solvent or vehicle such as a triglyceride oil or polyethylene glycol, and encapsulated in a gelatin capsule. An optimal drug form for this dosage form is one which has a high solubility in an appropriate soft-gel vehicle. In general, a drug form which is more soluble in a triglyceride oil will be less soluble in water. Identification of an appropriate drug form for a soft-gel dosage form requires study of various salts, polymorphs, crystalline, and noncrystalline forms.
Thus, it can be seen that the desired solubility of a drug form depends on the intended use, and not all drug forms are equivalent.
For a drug form to be practically useful for human or animal therapy, it is desirable that the drug form exhibit minimal hygroscopicity. Dosage forms containing highly hygroscopic drugs require protective packaging, and may exhibit altered dissolution if stored in a humid environment. Thus, it is desirable to identify nonhygroscopic crystalline salts and polymorphs of a drug. If a drug is noncrystalline, or if a noncrystalline form is desired to improve solubility and dissolution rate, then it is desirable to identify a noncrystalline salt or form which has a low hygroscopicity relative to other noncrystalline salts or forms.
A drug, crystalline or noncrystalline, may exist in an anhydrous form, or as a hydrate or solvate or hydrate/solvate. The hydration state and solvation state of a drug affects its solubility and dissolution behavior.
The melting point of a drug may vary for different salts, polymorphs, crystalline, and noncrystalline forms. In order to permit manufacture of tablets on commercial tablet presses, it is desirable that the drug melting point be greater than around 60° C., preferably greater than 100° C. to prevent drug melting during tablet manufacture. A preferred drug form in this instance is one that has the highest melting point. In addition, it is desirable to have a high melting point to assure chemical stability of a solid drug in a solid dosage form at high environmental storage temperatures which occur in direct sunlight and in, geographic areas such as near the equator. If a soft-gel dosage form is desired, it is preferred to have a drug form which has a low melting point, to minimize crystallization of the drug in the dosage form. Thus, it can be seen that the desired melting point of a drug form depends on the intended use, and not all drug forms are equivalent.
When a drug's dose is high, or if a small dosage form is desired, the selection of a salt, hydrate, or solvate affects the potency per unit weight. For example, a drug salt with a higher molecular weight counterion will have a lower drug potency per gram than will a drug salt with a lower molecular weight counterion. It is desirable to choose a drug form which has the highest potency per unit weight. The method of preparation of different crystalline polymorphs and noncrystalline forms varies widely from drug to drug. It is desirable that minimally toxic solvents be used in these methods, particularly for the last synthetic step, and particularly if the drug has a tendency to exist as a solvate with the solvent utilized in the last step of synthesis. Preferred drug forms are those which utilize less toxic solvents in their synthesis.
The ability of a drug to form good tablets at commercial scale depends upon a variety of drug physical properties, such as the Tableting Indices described in Hiestand H, Smith D. Indices of tableting performance. Powder Technology, 1984; 38:145-159. These indices may be used to identify forms of a drug, e.g. of atorvastatin calcium, which have superior tableting performance. One such index is the Brittle Fracture Index (BFI), which reflects brittleness, and ranges from 0 (good—low brittleness) to 1 (poor—high brittleness). Other useful indices or measures of mechanical properties, flow properties, and tableting performance include compression stress, absolute density, solid fraction, dynamic indentation hardness, ductility, elastic modulus, reduced elastic modulus, quasistatic indentation hardness, shear modulus, tensile strength, compromised tensile strength, best case bonding index, worst case bonding index, brittle/viscoelastic bonding index, strain index, viscoelastic number, effective angle of internal friction (from a shear cell test), cohesivity (from a powder avalanche test), and flow variability. A number of these measures are obtained on drug compacts, preferably prepared using a triaxial hydraulic press. Many of these measures are further described in Hancock B, Carlson G, Ladipo D, Langdon B, and Mullarney M. Comparison of the Mechanical Properties of the Crystalline and Amorphous Forms of a Drug Substance. International Journal of Pharmaceutics, 2002; 241:73-85.
Drug form properties which affect flow are important not just for tablet dosage form manufacture, but also for manufacture of capsules, suspensions, and sachets.
The particle size distribution of a drug powder can also have large effects on manufacturing processes, particularly through effects on powder flow. Different drug forms have different characteristic particle size distributions.
From the above discussion, it is apparent that there is no one drug form which is ideal for all therapeutic applications. Thus it is important to seek a variety of unique drug forms, e.g. salts, polymorphs, noncrystalline forms, which may be used in various formulations. The selection of a drug form for a specific formulation or therapeutic application requires consideration of a variety of properties, as described above, and the best form for a particular application may be one which has one specific important good property while other properties may be acceptable or marginally acceptable.
The present invention answers the need by providing novel forms of atorvastatin magnesium. Thus the present invention provides new forms of atorvastatin magnesium designated Forms A, B, C, D, E, and F. The new forms of atorvastatin magnesium disclosed in the present application offer the advantage of high water solubility. This is an advantage for immediate release dosage forms since such forms need to be fully dissolved in the stomach before passing into the digestive tract.